1. Field of the Invention
The present invention relates to a metallized silicon nitride ceramic having a metallizing layer of high bonding strength on the surface of a highly heat-conductive, mechanically-strong silicon nitride ceramic; to a fabricating process thereof; and to a metallizing composite for the process. In the invention, the term xe2x80x9csilicon nitride ceramic(s)xe2x80x9d as well as the term xe2x80x9cSi3N4 ceramic(s)xe2x80x9d includes Si3N4-based ceramic(s), Si3N4-glass-based ceramic(s), and Si6xe2x88x92xAlxOxN4xe2x88x92x (Sialon)-based ceramic(s).
2. Description of the Background Art
Highly heat-conductive materials also superior in electrical insulation, mechanical strength, and antiwear quality are needed as a sliding part in mechanical equipment or as a circuit substrate in electronic devices. Ceramics are used in these fields as a material that meets the requirements.
An alumina, one of the ceramic materials, is usually used in these fields. Although superior in electrical insulation and antiwear quality, the alumina has low thermal conductivity and hence is not suited for use in electronic devices or in a sliding part of mechanical equipment where heat must be rapidly dissipated to prevent excessive temperature increase.
Recently, an aluminum nitride having a high thermal conductivity, therefore having high capability in heat dissipation, is increasingly used. But the aluminum nitride has poor antiwear quality and mechanical strength, hence has low reliability as a structural member. In response to this problem, Si3N4 ceramics having a high mechanical strength and high thermal conductivity are being developed. And there is a high possibility to solve the problem as a ceramic package comprising a Si3N4-based ceramic with a bending strength of 30 kg/mm2 or more as disclosed in the unexamined published Japanese patent application Tokukaihei 4-125950.
When a highly heat-conductive Si3N4 ceramic is used as a semiconductor package or circuit substrate or is brazed to a metal part as a structural member of a machine, the surface of the ceramic must be metallized. For example, the unexamined published Japanese patent application Tokukaihei 9-69672 discloses several methods of surface metallization for Si3N4-based ceramics, such as a directly-uniting method of a highly heat-conductive Si3N4-based ceramic substrate with a sheet of copper circuit, an active-metal method to unite an Si3N4-based ceramic substrate with a sheet of metal circuit, and a method to metallize the surface of an Si3N4-based ceramic substrate by a high-melting-point metallization method.
However, the above-mentioned methods are incapable of forming a highly reliable, strongly-bonded metallizing layer on the surface of an Si3N4-based ceramic. Furthermore, the directly- uniting method of an Si3N4-based ceramic substrate with a sheet of copper circuit and the active-metal method disclosed in the Tokukaihei 9-69672 are limited in the application of the metallized Si3N4-based ceramics.
The high-melting-point metallization method disclosed in the Tokukaihei 9-69672, which bakes a metallizing composite consisting mainly of a high-melting-point metal and titanium or its alloys onto the surface of an Si3N4-based ceramic, gives a relatively weak bonding between the Si3N4-based ceramic and the metallizing layer. Therefore, products fabricated by this method are unreliable for adequate use in heavy-duty bonded parts, though this method provides adequate bonding strength for a copper circuit substrate such as disclosed in the Tokukaihei 9-69672.
To overcome the above-mentioned difficulties, the present invention provides a highly reliable metallized Si3N4 ceramic having a metallizing layer of high bonding strength on the surface of a highly heat-conductive, mechanically-strong Si3N4 ceramic; a fabricating process thereof; and a metallizing composite for the process.
For the above purpose, the metallized silicon nitride ceramic of the invention specifically has a metallizing layer, which contains 0.01 to 20% silicon by weight in terms of silicon, formed on the entire or a part of the surface of a silicon nitride ceramic having 0.01 to 10% free silicon by weight.
In the metallized silicon nitride ceramic of the invention, it is preferable that the silicon nitride ceramic have a thermal conductivity of 50 W/mK or more and a bending strength of 600 MPa or more, and that the metallizing layer contain 0.01 to 5% silicon by weight in terms of silicon, and that the contained silicon be free silicon.
A fabricating process for the metallized silicon nitride ceramic of the invention specifically comprises that first, silicon powders having an average particle size of 5 xcexcm or less and powders of rare earth compounds are mixed to form a compact; second, the compact is heated in a non-oxidizing atmosphere including nitrogen to produce a silicon nitride ceramic having 0.01 to 10% unnitrided, or free, silicon by weight; and third, a metallizing paste is applied on the entire or a part of the surface of the silicon nitride ceramic to be baked in a non-oxidizing atmosphere so that a metallizing layer containing 0.01 to 20% silicon by weight in terms of silicon is formed.
The other fabricating process for the metallized silicon nitride ceramic of the invention specifically comprises that first, silicon powders having an average particle size of 5 xcexcm or less and powders of rare earth compounds are mixed to form a compact; second, a metallizing paste is applied on the entire or a part of the surface of the compact; and third, the compact is sintered with heat in a non-oxidizing atmosphere including nitrogen to produce a silicon nitride ceramic having 0.01 to 10% unnitrided, or free, silicon by weight and to form at the same time a metallizing layer containing 0.01 to 20% silicon by weight in terms of silicon on the surface of the ceramic.
In either of the above-mentioned processes, the metallizing paste comprises a metallizing composite and a vehicle, which includes a binder resin and a solvent. In the invention, the metallizing composite has a metal component as its main ingredient and may include 0.01 to 20% silicon by weight in terms of silicon.
According to the present invention, a highly reliable metallized Si3N4 ceramic may be produced, which comprises a highly heat-conductive, high strength bending and wear-resistant Si3N4 ceramic having a strongly bonded metallizing layer thereon.
The metallized SiN3N4 ceramic of the invention is useful as a ceramic circuit substrate for transistors, a ceramic package, and other parts of semiconductor devices or as a machine""s structural member in combination with a metal part. The ceramic may serve additionally to improve the performance of these devices and machines.
Efforts were made to seek a means for improving the bonding strength between an Si3N4 ceramic and a metallizing layer. Finally it was found that high bonding strength is attainable by combining an Si3N4 ceramic having free silicon with a metallizing layer having free silicon and other silicon-containing components. The end result was a highly heat-conductive metallized Si3N4 ceramic with exceptionally high mechanical strength.
In the invention, it is imperative that an Si3N4 ceramic to be metallized contains 0.01 to 10% free silicon by weight. The free silicon in the Si3N4 ceramic plays an important role in improving the bonding strength between the ceramic and the metallizing layer having silicon components. If the content is less than 0.01% by weight, the bonding between the ceramic and the metallizing layer is weakened; if more than 10% by weight, the mechanical strength and heat-resisting property of the Si3N4 ceramic itself decreases.
It is desirable that the Si3N4 ceramic have a thermal conductivity of 50 W/mK or more and a bending strength of 600 MPa or more. To obtain sufficient heat-dissipating power as a circuit substrate, a ceramic package, and other electronic parts or as a sliding part in machines, it is desirable that the ceramic have a thermal conductivity of 50 W/mK or more, preferably 100 W/mK or more. To obtain sufficient mechanical strength as a reliable structural member of machines, it is desirable that the ceramic have a bending strength of 600 MPa or more.
It is also imperative that the metallizing layer on an Si3N4 ceramic not only have a metal component as its main ingredient but also contain 0.01 to 20% silicon by weight in terms of silicon, preferably 0.01 to 5% by weight. If the content is less than 0.01% by weight, the bonding between the ceramic and the metallizing layer is weakened; if more than 20% by weight, the mechanical strength and heat-resisting property of the metallizing layer itself decreases.
The silicon component included in the metallizing layer may comprise a compound such as Si3N4, SiC, or a silicide as well as free silicon. Free silicon is especially preferable because it exhibits superior effects on such properties as bonding strength. The silicon component may be added directly into a metallizing paste or metallizing composite, or the free silicon included in the Si3N4 ceramic may be introduced into the metallizing layer by diffusion when the metallizing layer is formed.
To fabricate the metallized Si3N4 ceramic of the invention, an Si3N4 ceramic, superior in many aspects as described above, having 0.01 to 10% free silicon by weight is first produced, and a metallizing paste is applied on the surface of the ceramic to be baked so that a metallizing layer is formed. As mentioned above, the required amount of free silicon may be introduced into the metallizing layer by diffusion from the Si3N4 ceramic or a predetermined amount of silicon component may be mixed with the metallizing paste beforehand.
However, it is usually time-consuming and costly to produce a highly heat-conductive Si3N4 ceramic which has simultaneously the required amount of free silicon. Hence, it is desirable that the fabricating process of the invention be applied as described below, which employs a reaction-sintering method.
The first fabricating process of the invention comprises that first, silicon powders having an average particle size of 5 xcexcm or less and powders of rare earth compounds are mixed to form a compact; second, the compact is heated in a non-oxidizing atmosphere including nitrogen to produce an Si3N4 ceramic having 0.01 to 10% unnitrided, or free, silicon by weight; and third, a metallizing paste is applied on the surface of the Si3N4 ceramic to be baked in a non-oxidizing atmosphere so that a metallizing layer is formed.
If the average particle size of the silicon powders used as the material exceeds 5 xcexcm, the nitriding reaction of the silicon is inhibited and a superior Si3N4 ceramic cannot be obtained. Of course, material silicon powders having an average particle size of more than 5 xcexcm can be crushed to reduce the particle size to 5 xcexcm or below before the compaction. Powders of rare earth compounds are added to facilitate the nitriding reaction and densifying process of the silicon, a preferred amount of addition being 1 to 15% by weight of the silicon powders. The rare earth compounds may be selected from the group including individual oxides of Ce, Sm, and La as well as from Y2O3. Compounds that transform to the rare earth compounds by heating may also be used.
The mixed body of powders of silicon and powders of rare earth compounds is compacted by a conventional method, and heated in a non-oxidizing atmosphere including nitrogen for nitriding and sintering. The suitable range of the heating temperature depends on the particle size and purity of the silicon. Generally, the desirable temperature lies between 1300 and 2000xc2x0 C. because the nitriding proceeds too slowly below 1300xc2x0 C. According to the first fabricating process of the invention, on the surface of the Si3N4 ceramic thus provided, a metallizing paste is applied with a designed contour to be baked in a non-oxidizing atmosphere so that a metallizing layer is formed.
The second fabricating process of the invention comprises that first, silicon powders having an average particle size of 5 xcexcm or less and powders of rare earth compounds are mixed to form a compact; second, a metallizing paste is applied on the entire or a part of the surface of the compact; and third, the compact is nitrided and sintered in a non-oxidizing atmosphere Including nitrogen to produce a silicon nitride ceramic having 0.01 to 10% unnitrided, or free, silicon by weight and to form at the same time a metallizing layer on the surface of the ceramic. In the second fabricating process, the material powders used and other conditions such as nitriding and sintering conditions are the same as those in the first fabricating process except that the metallizing layer is formed at the same time when the compact is nitrided and sintered.
In both the first and second fabricating processes, it is essential that the unnitrided, or free, silicon of 0.01 to 10% by weight remain in the completed Si3N4 ceramic and that silicon of 0.01 to 20% by weight be included in the metallizing layer. Hence, it is important to control the heating temperature and time during the reaction-sintering and baking.
As for the control of the amount of free silicon in the Si3N4 ceramic, it is preferable that first, the temperature for nitriding and sintering be kept between 1300 and 1400xc2x0 C. for a relatively long time so that the free silicon of 0.01 to 10% by weight remains unnitrided, and second, the temperature is raised to and kept at 2000xc2x0 C. or lower for a short time so that the sintering proceeds further to densify the ceramic. Especially in the second fabricating process, care must be taken so that the excessive diffusion of free silicon from the Si3N4 ceramic to the metallizing layer does not occur because the metallizing layer is formed at the same time when the heating for nitriding and sintering is carried out for the ceramic.
The free silicon of 0.01 to 20% by weight in the metallizing layer may either be introduced by diffusion from the Si3N4 ceramic or be included in the metallizing paste beforehand by adding a predetermined amount, roughly 0.01 to 20% by weight, of silicon or silicon-containing substances such as silicon compounds that transform into silicon by heating.
More specifically, since free silicon Is contained in the Si3N4 ceramic, the free silicon may be diffused into the metallizing layer when heating is carried out for nitriding and sintering or baking so that the content of free silicon in the metallizing layer exceeds 0.01% by weight. This means that an ordinary metallizing paste comprising a metal component as the main ingredient and a vehicle may be used. In this case, the heating temperature and time must be controlled to satisfy the above-mentioned condition of silicon content in the metallizing layer.
The other method is to use the metallizing composite of the invention, which comprises a metal component as the main ingredient and a silicon component of 0.01 to 20% by weight in terms of silicon. The composite is included in a metallizing paste together with a vehicle. Since the necessary amount of silicon is already included in the metallizing layer, the heating temperature and time must be controlled so that the silicon content in the metallizing layer does not exceed 20% by weight through the diffusion of silicon from the ceramic.
The metallizing composite of the invention mainly comprises at least one metal selected from the group consisting of gold, silver, copper, tungsten, and molybdenum. The selection for the most suitable metal or metals is based on the necessity and required reliability of brazing, the required electric properties, and other specific conditions. Gold, silver, and copper can be used for a relatively low-temperature metallization, and a high-melting-point metal such as tungsten and molybdenum is used for a high-temperature metallization. An active metal such as titanium may also be added to tungsten or molybdenum.
The metallized Si3N4 ceramic of the invention enables a highly reliable bonding with various metals or ceramic parts through the metallizing layer on the surface thereof. The ceramic is particularly useful as a ceramic circuit substrate, a ceramic package, and other parts of semiconductor devices or as a structural member brazed with a metal part in various machines. In applications such as a ceramic circuit substrate, a bonding strength of 20 to 30 MPa is required between the ceramic substrate and the metallizing layer that forms a part of an electric circuit, preferably 60 MPa or more to secure reliability In actual use. Such a requirement is fulfilled by the metallized Si3N4 ceramic of the invention.